The breast cancer tumor suppressor protein BRCA1 participates in multiple aspects of DNA damage responses. Its function in DNA double-strand break (DSB) repair, especially in homologous recombination (HR)-mediated DSB repair is highly relevant to its role in the maintenance of genome stability and tumor suppression. However, the molecular mechanism underlying this DSB repair function of BRCA1 is still not clear. Our proposed studies will focus on the investigation of how BRCA1 and the BRCA1-association protein CtIP participate in DSB repair, which will help to elucidate the mechanisms underlying BRCA1 function in tumor suppression. First, we will study cell cycle-dependent phosphorylation of CtIP and probe the biological significance of these phosphorylation events in HR-mediated DSB repair. Since CtIP is a critical player to bridge the interaction of BRCA1 with the repair protein complex Mre11/Rad50/Nbs1, understanding cell cycle-mediated regulation of CtIP is important for determining the exact role of BRCA1 in the activation of HR. Second, we will examine the enzymatic activities of CtIP and the BRCA1/CtIP/MRN complex on DNA, which will reveal the biochemical basis for the function of BRCA1 in DSB repair. Third, we will investigate the role of BRCA1 and CtIP in the repair of DSBs caused by replication fork collapse, and probe the importance of this repair activity in fragile site protection. Fourth, we will analyze the recruitment of CtIP to DSBs through a specific protein-protein interaction, which may promote the complex formation of BRCA1/CtIP/MRN at chromatin proximal to DSBs, thus facilitating HR-mediated DSB repair. Together, these studies will help to elucidate the molecular mechanisms underlying the critical function of BRCA1 and CtIP in the maintenance of genome stability and will provide significant insights into how BRCA1 functions as a tumor suppressor. The findings from these studies will also open new avenues towards novel therapeutic interventions for breast cancer prevention and treatment.